Ammo can be taken care of by keeping all of it in ammo cans with good seals. I tend to pack mine in the winter, up stairs during cold weather. Thus with forced air heat the humidity is low.
Would get a de-humidifier for the area of the basement which is the gun room / area. Also keep an emergency sump pump and hose around, just in case.
Would think about putting a heater (light bulb or a little safer is the metal cased golden rod) in your safe and possible added indicator desicant (at least tells when humidity is higher for a long period of time).
While a little easy to tip / drag, one could think about making a base for the safe to keep of the cold floor and above any light flooding (same for spacing away from block / cold walls).
Also would wander over to Brownells and look / request a paper catalog. Also consider increasing oiling, inpartiuclar the bore. Insert my standard plug for Brownell's Rust Preventive No. 2 here [;)]